Field of the Invention
The invention relates to a bipolar plate for a fuel cell, to a fuel cell having such a bipolar plate and to a motor vehicle which has such a fuel cell.
Fuel cells use the chemical conversion of a fuel with oxygen to water in order to generate electrical energy. For this purpose, fuel cells comprise, as core component, what is termed the membrane electrode assembly (MEA) which is an assembly of an ion-conducting, in particular proton-conducting, membrane, and in each case an electrode (anode and cathode) arranged on either side of the membrane. When the fuel cell is in operation, the fuel, in particular hydrogen H2 or a hydrogen-containing gas mixture, is supplied to the anode, where electrochemical oxidation takes place with release of electrons (H2→2 H++2 e−). The membrane, which separates the reaction spaces gas-tightly from one another and electrically insulates same, transports the protons H+ (in a water-bound or water-free manner) from the anode space into the cathode space. The electrons e− provided at the anode are fed to the cathode via an electric line. The cathode is supplied with oxygen or an oxygen-containing gas mixture, and therefore a reduction of the oxygen takes place with the electrons being absorbed (½ O2+2 e−→O2−). At the same time, said oxygen anions react in the cathode space with the protons transported via the membrane, with water being formed (2 H++O2−→H2O).
Generally, the fuel cell is formed by a multiplicity of stacked membrane electrode assemblies, the electrical power of which adds up. In a stack of fuel cells, a respective bipolar plate is arranged between two membrane electrode assemblies, said bipolar plate serving firstly for supplying the process gases to the anode or cathode of the adjacent membrane electrode assemblies and also for discharging heat. In addition, bipolar plates are composed of an electrically conductive material in order to produce the electrical connection. They therefore have the triple function of supplying process gas to the membrane electrode assemblies, of cooling and of electrical connection.
Variously constructed bipolar plates are known. Fundamental aims in the design of bipolar plates are reduction in weight, reduction in construction space, reduction in costs and increase of the power density. These criteria are important in particular for the mobile use of fuel cells, for example for the electromotive traction of vehicles.
Description of Related Art
Bipolar plates for fuel cells have a customarily centrally arranged active region which is connected to the catalytic electrodes of the membrane electrode assembly and at which the actual fuel cell reactions take place. For this purpose the active region has an open anode gas flow field on the anode side and an open cathode gas flow field on the cathode side. The anode gas flow fields and cathode gas flow fields are generally designed in the form of groove-like channels. However, open-pored/porous structures are also known (US 2011/0039190 A1). In addition, the active region has a closed coolant flow field, wherein the latter is customarily designed in the form of enclosed channels.
In order to supply the active region with the corresponding operating materials, bipolar plates also have through openings for operating materials, namely in each case two anode gas main channels for supplying and discharging the anode gas, two cathode gas main channels for supplying and discharging the cathode gas, and two coolant main channels for supplying and discharging the coolant. In the stack of fuel cells, said through openings for operating materials lie congruently on one another, and therefore they form main supply channels, which pass through the entire stack, for the corresponding operating materials. Customary bipolar plates furthermore have inactive supply regions which serve substantially for the connection between the through openings for operating materials and the corresponding flow fields of the active region. The supply regions here each comprise anode gas channels which are connected in a fluid-conducting manner firstly to the anode gas main channel and secondly to the anode gas flow field of the active region. The inactive supply regions furthermore have cathode gas channels which are connected in a fluid-conducting manner firstly to the cathode gas main channel and secondly to the cathode gas flow field of the active region. Furthermore, the inactive supply region comprises coolant channels which are connected in a fluid-conducting manner firstly to the coolant main channel and secondly to the coolant flow field.
Examples of a bipolar plate according to the above description are disclosed in US 2006/0127706 A1 and DE 102007 008 214 A1. The through openings for operating materials are arranged here in each case on the two mutually opposite narrow sides of the bipolar plates, wherein the coolant main channel is in each case positioned substantially between the anode gas main channel and the cathode gas main channel.
A disadvantage of the known bipolar plates is that very different operating pressures prevail within the anode gas channels and particularly within the anode flux field of the active region. In particular, corner and edge regions of the active surface of the bipolar plate are frequently undersupplied. This problem of the different supply of anode gas to the active surface is particularly apparent in the case of flux fields having interrupted channels, in which the individual anode channels are connected laterally to one another. The high pressure differences cause significant transverse flows within the anode gas flux field and particularly great inhomogeneity here.
A further problem of the known bipolar plates is product water which arises on the cathode side, diffuses through the polymer electrolyte membrane and therefore passes onto the anode side. The water in the anode gas flow field and in the anode gas channels of the supply regions freezes there at low temperatures after the fuel cell has been switched off. This may cause clogging of the channel structures which cannot be freed by the comparatively low operating pressure of the anode gas. If such a bipolar plate is used in a fuel cell of a motor vehicle, after a frost start the water therefore has to be first of all thawed by the coolant. The operating readiness of the fuel cell is thereby delayed.
US 2007/0202383 A1 discloses confronting the problem of non-uniform distribution of anode gas by the anode gas channels of the active region of the bipolar plate branching into a different number of channels.